Semiconductor laser devices are particularly useful for transmitting information in the form of light pulses or intensity-modulated light along optical fibers. The desirability of transmitting information by light is directly attributable to the large bandwidths, i.e., the large amount, at large speeds (only fractions smaller than speed of light in vacuum) that can be transmitted compared to that using electrical signals. Electrical signals are inherently limited in frequency and speed due to electrical loading, mainly due to capacitive effects, and limitations on design of device drivers due to needs of conserving power. In order to take advantage of the increased performance offered by light transmission systems, small and efficient laser structures must be developed which can transmit the light reliably and cheaply.
Laser structures which have been built in the prior art to accomplish the goals described above fall roughly into two categories. One category of lasers emits light normal to the surface of a semiconductor wafer. The light emitted from these surface emitting type structures is focused into an optical fiber which must be aligned normal to the surface of the semiconductor wafer or the light must be reflected in the direction desired. The mechanical alignment of the optical fiber to the surface emitting laser is a difficult task to accomplish cheaply and reliably.
In addition, most surface emitting lasers have current injected into tile optical cavity of the device through the same material layers through which light is emitted. Such surface-emitting lasers have a layer structure requiring high reflectivity mirrors, which are by necessity periodically varying quarter-wave layers of materials with different indexes of refraction. The interfaces between such layers are generally abrupt and have high resistance to motion of carriers. This leads in turn to high resistance for current injection into the optical cavity and hence high power dissipation. The increased use of power places many limitations, such as decreased use with other low power electronic devices, on the utility of these lasers. Although, the surface emitting laser has many technical problems, the surface emitting laser has several significant advantages, such as low threshold current due to the use of a small active gain volume of the optical cavity together with high reflectivity and low optical loss mirrors, low divergence angle of emitted light due to the use of broad dimensions, and single-mode operation due to broad mode-to-mode spacing and large changes in reflectivity between the mode wavelengths. Moreover, the surface emitting lasers can be built using conventional semiconductor techniques which are well understood and form the basis for an inexpensive device.
Another category of semiconductor laser is an edge emitting semiconductor laser. In this type of device, multiple layers of materials are grown on a substrate which will later form the laser device. In the most common form, the laser device is actually formed by cleaving out an optical cavity from the substrate. The cleaved edges of the optical cavity generally have sufficient reflectivity to permit the lasing action to occur. The reflectivity of the cleaved surfaces can be enhanced through the deposition of mirrors or complex Bragg reflectors onto the cleaved surfaces. The problem with this edge emitting device is that the mode spacings are short because of the use of a large optical cavity, leading to multi-mode operation and a larger threshold current due to the use of a larger active-gain volume in the optical cavity. In addition, once the optical cavity has been cleaved from the substrate, the subsequent processing of the laser device becomes very labor intensive and therefore expensive. The edge emitting device which has a long optical cavity typically has large power dissipation because of the long cavity. Again, as in the surface emitting device, a large power dissipation is a significant shortcoming of the device. Moreover, it is difficult to integrate the cleaved laser with other microelectronic structures to manufacture an inexpensive information processing system. In general, neither of the common forms of the surface emitting lasers or the edge emitting lasers built to date, possess the optical and electrical properties necessary for optimal electronic communication and/or low manufacturing costs necessary for the integration of laser technology into digital communication.